1. Field of the Invention
The present invention relates to a transmitting apparatus which transmits contents, a receiving apparatus which receives contents, and a content transmitting method.
2. Description of the Related Art
In recent years, along with the spread of computer networks such as a broadband network and wireless LAN, digital information apparatuses or digital household appliances having a communication function have prevailed. Digital broadcasting-compatible TV sets, set-top boxes, DVD recorders, and the like will become more popular. If these digital information apparatuses and the like are connected to networks, users can enjoy contents via the networks.
The contents include various types of digital data, e.g., moving image data and audio data complying with MPEG2 or MPEG4, or document data such as text data and image data. Advantageously, the contents including such digital data can be readily copied without degradation. However, caution must be exercised for the copyrights on the contents.
A mechanism of transmitting/receiving copyright protected contents between transmitting and receiving apparatuses via a network has been proposed (see, e.g., a document available at www.dtcp.com (a PDF file linked to the item “Other DTCP Informational Specifications” in the above URL)).
A copyright protected content has copy control information, and a copy or move of the content is controlled according to the copy control information. For example, copy control information “No-more-copies” represents that no more copies can be made but a move is permitted.
Various methods are available for moving a content with the copy control information “No-more-copies” while protecting the copyright on the content. For example, a method can be implemented by the following three steps.
(1) Authentication/key exchange processing is executed between transmitting and receiving apparatuses.
(2) A content is copied from the transmitting apparatus to the receiving apparatus in an “unusable state”. At this time, a content in a “usable state” exists in the transmitting apparatus.
(3) The content in a “usable state” in the transmitting apparatus is set to an “unusable state”, and the content in an “unusable state” in the receiving apparatus is set to a “usable state”. This processing is referred to as content use right transfer processing.
This method is called a transaction move, and the above three steps constitute one transaction.
In the mechanism of transmitting/receiving copyright protected contents between transmitting and receiving apparatuses via a network, there may be a rule that “the receiving apparatus must discard a decryption key if it does not use the decryption key for a predetermined period of time”.
Consider a move of an edit content containing a plurality of portions with different pieces of copy control information. Such content is, for example, a content obtained by concatenating content 1 with copy control information “No-more-Copies”, content 2 with copy control information “Copy-free”, and content 3 with copy control information “No-more-Copies”.
First, authentication/key exchange processing is executed between the transmitting and receiving apparatuses to share a secret key to be used for encryption and decryption. Assume that an edit content containing a portion not requiring encryption with copy control information “Copy-free” is moved. The receiving apparatus discards the secret key when the predetermined period of time elapses after the start of receiving the portion not requiring encryption. If the transmitting apparatus continuously transmits a portion requiring encryption with copy control information “No-more-Copies”, it encrypts the portion requiring encryption using the secret key and transmits it to the receiving apparatus. Since the receiving apparatus has discarded the secret key, it cannot decrypt the subsequent portion requiring encryption.
In case of moving a content, it is often difficult for the transmitting apparatus to determine whether the receiving apparatus is an authentic receiving apparatus which shares the secret key. Therefore, even if the receiving apparatus requests the authentication/key exchange processing again to share the secret key with the transmitting apparatus, it is difficult for the receiving apparatus to acquire the secret key.
The following method is also available.
(1) A transmitting apparatus divides an edit content.
(2) The divided contents are moved from the transmitting apparatus to a receiving apparatus as separate contents.
(3) The receiving apparatus concatenates the divided contents to reconstruct the original content.
In this method, however, it is impossible to smoothly concatenate the divided contents.
Note that “it is impossible to smoothly concatenate the divided contents” means that when video data are played back, it is impossible to smoothly play back the contents due to a frame loss. If, for example, an MPEG2 content is divided at a Close I picture as an ideal position and then the divided contents are concatenated, it is possible to smoothly play back the content. If, however, the content is divided at a position other than the ideal position, some of P pictures and B pictures at the start portion of the second divided content are discarded. Consequently, even if the divided contents are concatenated, it is impossible to smoothly play back the content without the discarded pictures.
In JP-A 2006-338779(KOKAI), to solve the above problems, management information is created before a content is divided, and the divided contents are smoothly concatenated using both the management information on the content prior to the division and management information on the divided contents. If, however, it is impossible to create management information in advance in terms of copyright protection, it is impossible to use this method and to smoothly concatenate the divided contents.
As described above, under the circumstances in which a secret key may be discarded due to copyright protection, it is difficult to reliably move a content from a transmitting apparatus to a receiving apparatus without losing part of the content.